Crossword Genius

Defence has no time for treachery (6)

Ross

I believe the answer is:

reason

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'defence' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for reason that I've seen before include "Motivation or cause" , "Sanity, the ability to think" , "Cause or motive" , "Employ logic" , "Justification or motive" .)

I've seen this clue in The Irish Times.
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